Method of rendering an aluminum-iron alloy ductile



2,801,942 Patented Aug. 6, 1957 METHQD @F RENDERENG AN ALUMINUM-IRONAlli)?! DUCTILE No Drawing. Application February 25, 1954, Serial No.412,963

15 Claims. (Cl. 1482) (Granted under Title 35, U. S. Code (1952), sec.2.66)

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

The present invention relates to a magnetic material and moreparticularly to a new and improved method of making the same intoductile and workable sheet and tape form.

Aluminum-iron alloy heretofore used exhibited excellent electrical andmagnetic properties. However, because of the inherent brittlecharacteristic thereof the material was not used extensively for thereason that it was impossible to be cold worked into sheet form withoutbreakage thereof.

The present invention contemplates the provision of a new and improvedmethod of making such material into a workable strip which issufficiently tough and ductile to be sheared into any desired shapewithout damage or breakage. Furthermore, the new and improved methodproduces a magnetic material having the desired magnetic characteristicsand which possesses isotropic magnetic properties and high bulkresistivity thereby preventing electrical losses. Moreover, inaccordance with the new and improved method it has been found that thematerial developed or grew its own insulatinglayer and thus in variousmagnetic applications, the usual insulating fabrication process could beeliminated. In certain other applications it has been found that themagnetic material produced by the new and improved method is farsuperior to the hot Worked aluminum-iron alloy now in use. Furthermore,in transformer cores of the type used in high frequency communicationinstruments, it has shown properties far superior to those shown by thesiliconiron cores now so extensively used in such instruments.Furthermore, while alloys of the aforesaid type have been heretoforeused no successful method has been devised for working such alloys intothin sheets or tapes having the desired ductility or malleability torender them commercially useful, particularly if such alloys containedaluminum in excess of In accordance with my discovery it is possible toreduce such sheets having up to 16% aluminum into relatively thin tapeshaving improved electrical characteristics and also reduce thebrittleness thereof sufiiciently to permit the tapes to be readily bentinto any desired shape without causing breakage thereof.

An object of the present invention is to improve the present methods ofmanufacturing Al-Fe sheets such that sheets having an aluminum contentin excess of 5% may be readily rolled into tape-like form without thelosses in yield now encountered because of the lack of ductility ormalleability of the sheets made under the present hot working methods.

Another object of the invention is to provide a method of making brittlealuminum-iron alloy into workable sheet or tape-like form.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description.

In the process of making the aluminum-iron alloys into workable stripsor tapes in accordance with the present invention, the iron is melted ina suitable furnace such, for example, as a high frequency furnace. Aftermelting the iron the aluminumis added thereto. After allowing sufiicienttime to permit proper mixing of the aluminum and iron the temperature isregulated and the melt is cast into a slab mold designed to produce afine, preferably an equiaxed grain structure. The slab is then reducedin thickness by hot rolling at temperatures between 1000 C.-1050 C. from1" to substantially 0.250, the last or final hot rolling operation to athickness of 0.125 being conducted at about 900 .C. in order to obtainfull benefit from the grain refinement occurring at the lowertemperature.

With reference to the article The lattice spacings of iron-aluminumalloys, by A. J. Bradley and A. H. Jay, Journal of Iron and SteelInstitute [London], pages 339 to 361 of volume [1932], and asillustrated in page 1161 of the Metals Handbook, 1948 edition, publishedby American Society for Metals, it has been determined that the regionof order-disorder transformation occurs in aluminum-iron alloys havingan aluminum content. of 10 to 20%. Therefore, alloys containing from 10to 20% aluminum have a tendency to order into an FesAl type lattice,which is believed to be mechanically softer than the disordered phase-However, although alloys containing 5 to 10% aluminum are not fullycharacterized by the FeaAl type. lattice, it was discovered that.working these alloys in the order-disorder temperature range enhancedthe ductility thereof and made feasible a reduction in thickness withoutbreakage. Thus if the aforesaid 0.125" material isfrolled on down in theorder-disorder temperature range, which lies in the neighborhood of 450C. or somewhat below 600 C., with the alloy maintained in this orderedor partly ordered condition it is sufficiently ductile to enable rollingthe material to any desired thickness. In the specification and theappended claims, the order-disordertemperature range shall be construedto include temperatures within the range of 450 C. to 600C. The coldrolling at elevated temperatures employed with the improved method wasconducted at average temperatures of 500 C., 550 C. and 575 C., thelatter being the most desirable and produced admirable results. Thetemperature variations of the furnace during the on and off cycles was550 C. to 600 C. whereupon an average temperature 575 C. was obtained.This technique was carried out by allowing the alloy to heat for a fewminutes between passes of the material through a suitable rolling millin order to maintain the alloy at a temperature of approximately 575 C.during the coldrolling process. A strip furnace may be employed, ifdesired, to speed up the aforesaid heating and rolling cycles. By thecold rolling method, sheets from 0.014" to 0.00035 in thickness wasproduced.

It has been found that sheets or strips approximately 0.007 thickproduced in the aforesaid manner and heated at 575 C. for a short periodof time gave admirable results. Also, sheets recrystallized with aminimum grain size have produced admirable results. These sheets may becut into strips /8" wide and the oxide coating removed to minimize wearduring the rolling operation as the strips were rolled at roomtemperature on a conventional mill to a thickness of substantially 0.002of an inch. When this operation has been completed the edges of the0.002 strips are removed yielding strips /4" wide and thereafter rollingoperation continued at room temperature until the strips are reduced inthickness to about 0.0005 of an inch thereby to form a thin tape. It hasbeen found that the strips may be successfully rolled into the aforesaidtape form without employing the annealing process and still maintain itsworkable characteristics.

It has been further discovered that the finished sheet rolled at 575 C.develops a surface coating of essentially aluminum oxide which is notelfectively reduced by a high temperature anneal in hydrogen. In fact,the quality of the aforesaid insulation appears to improve when thesheet is subject to the high temperature hydrogen anneal.. Furthermore,if desired, heavier coatings of aluminum oxide may be formed on theaforesaid sheets by annealing first with wet hydrogen followed byannealing with dry hydrogen. Moreover, in instances where the oxidecoating has been removed from the tape prior to additional rolling itmay be preoxidized in a conventional furnace to restore the coatingwhich makes it admirably suited for winding the tape into a toroidalcore, it being understood that the thickness of the oxide insulatingcoating will be a function of the temperature, time, and type ofatmosphere employed. The advantages of the thin surface oxide coatingespecially on the thin tape will obviously facilitate stacking of thetape since the tape is well insulated by the aforesaid coating and thusduring a stacking operation the usual insulating spaces hereto fore maybe eliminated.

Furthermore, it has been found by actual test that the magneticproperties of the material produced by the aforesaid method andcontaining nominally 16% aluminum and 84% iron is far superior inmagnetic properties found in material produced by the hot rolled methodalone which contains like amounts of the aforesaid alloys. For example,the magnetic properties obtained in a core structure employing magneticsheet produced in accordance with the present invention wherein thesheet is 0.014 of an inch thick with the ring laminations (2" O. D. x1%" I. D.) is set forth in the following example: ,u =115,880, ,u,=2,778, Hc=0.024, Br=4,192, and Bm=7,608 in comparison with the magneticproperties of a core structure produced in accordance with the aforesaidhot rolled method and set forth in the following example: ,um:55,000, ,u=3,100, Hc=0-O4, Br=2,100.

Furthermore, the high resistivity of 140-150 microohmcm. found in thenew and improved magnetic material is substantially 3 times greater thanthat found in silicon-iron alloys heretofore used. Moreover the corelosses in the improved magnetic material is substantially small comparedto losses of commercial silicon-iron alloys. For example: a 60 cyclecore loss of 0.04 watt/lb. for 0.014 thick material at a flux density of5000 gauss is substantially the value of the core losses suffered in asilicon-iron core tested under identical conditions. The improved powermaintaining properties and high mechanical hardness of the magneticmaterial produced in the new method are characteristics which made itadmirably suited for use in recording heads wherein resistance toabrasion is essential.

It will be understood that the material produced by the aforesaid methodis not highly grain oriented, in reality the material is magneticallyisotropic in contradistinction to the anisotropic material produced bythe silicon-iron alloys. Furthermore, the present invention does notrelate to austenitic alloys. The high Al-Fe series found in theaforesaid material is body centered cubic in crystal structure at highand low temperatures. Moreover, it is well known to those skilled in theart that the crystal structure of material directly aifects workabilitythereof in response to both heat and cold. Cold rolling denotes coldworking of the alloy at a temperature below the recrystallizationtemperature, whereas hot rolling denotes hot working the alloy at atemperature above the recrystallization temperature. In hot rollingprocess, for example, the metal is recrystallized, while in cold rollingprocess, the crystal structure remains the same and the crystals areelongated. Furthermore, it will be understood that in the aforesaidprocess of reheating to the order-disorder temperature range bringsabout some ordering of the basic Al-Fe alloy lattice thereby to producea ductile structure which may be cold rolled to a thin tape having athickness of 0.00035 of an inch, if desired.

The aforesaid new and improved method produces a magnetic materialhaving high resistance characteristic to corrosion particularly whensubjected to strong oxidizing solutions such, for example, asconcentrated HNOs and also to atmospheric oxidation at elevatedtemperatures and room temperatures.

An alternate method for reducing the aluminum-iron alloy to a thicknessof 0.00035 of an inch is as follows:

Roll the material at 575 C. from 0.125 of an inch to 0.028 of an inch;anneal at 575 C. for a short time to produce partial ordering thereof;roll from 0.028 of an inch to 0.014 of an inch at room temperature andthereafter anneal the 0.014 of an inch material at 575 C. for a shorttime; roll at room temperature from 0.014 of an inch to 0.007 of an inchand anneal the strip of 0.075" material for a short time at 575 C.; thefinal operation consisting of rolling the strip of 0.007" material to athickness of 0.00035 of an inch at room temperature.

The following method has also produced admirable results:

Roll the material to 0.014 of an inch by the hot and subsequent coldrolling process hereinbefore described in obtaining a sheet of 0.014" inthickness; followed by alternately heating the material at 575 C. for 5minutes and rolling the material at room temperatures with one or twolight passes through a suitable mill until the desired thickness thereofhas been obtained. This method indicates that the 5 minute heating at575 C. is suflicient to produce enough softening to allow the rolling tocontinue until the desired thickness of the material is obtained.

From the foregoing, it will be apparent that a new and improved magnetictape and method of making the same has been discovered wherein the tapeis characterized by a tough fibrous structure making it sufficientlyductile to be formed into suitable shapes without breakage or damagethereto and which possesses superior magnetic properties, low powerlosses, and high resistance to corrosion. Obviously many modificationsand variations ofthe present invention are possible in the light of theabove teachings. It is therefore to be understood that within the scopeof the appended claims the invention may be practiced otherwise than asspecifically described.

What is claimed as new and desired to be secured by Letters Patent inthe United States is:

1. The method of making ductile sheets or tape-like magnetic strips froman alloy having nominally 16% aluminum and 84% iron, which consists offorming the alloy into slabs, hot working the slab above therecrystallization temperature to a predetermined thickness, and coldworking the sheets in the order-disorder temperature range to a secondpredetermined thickness less than said first mentioned predeterminedthickness.

2. The method of making ductile sheets or tape-like magnetic materialsfrom an alloy containing from 5 to 16% aluminum and to 84% iron, whichconsists of forming the alloy into relatively thick sheets, cold workingthe sheets to a thickness of about 0.028" at a temperature of 575 C.,reducing the alloy to a thickness of 0.007 of an inch by subjecting thealloy sheets to alternate operations of rolling at room temperature andheating to a temperature within the order-disorder range, and thereafterrolling the alloy sheets at room temperature to a thickness of 0.00035of an inch.

3. The method of making ductile sheets or tape-like magnetic strips froman alloy containing from 5 to 16 percent aluminum and the remainder ironwhich consist of forming the alloy into relatively thick slabs, hotworking the slabs into sheets of a first predetermined thickness,

cold Working the sheets to a second predetermined thickness at atemperature of about 575 C., maintaining the sheets at said temperaturefor a short time, and thereafter reducing the thickness of the sheets byalternately heating and cold working the sheets.

4. In a method of making isotropic magnetic material in sheet ortape-like form, the steps including hot rolling an aluminum iron alloyof to 16 percent aluminum content and about 1 thick to a thickness ofabout 0.125" at a temperature of substantially 1000 C., cold rolling theslab at a temperature of about 575 C. to a thickness of about 0.007",heating the sheet at 575 C. for a short time, and thereafter rolling thesheet at room temperature to a thickness of substantially 0.002 of aninch.

5. In a method of making magnetic material into tapelike form, the stepsincluding hot rolling aluminum-iron slabs containing from 5 to 16percent aluminum into strips at a temperature of about 1000 C., coolingthe heated strips to below the recrystallization temperature, coldrolling the strips below the recrystallization temperature intotape-like form, heating said tape to about 575 C., and further rollingthe tape at room temperature until the desired thickness is obtained.

6. The method of claim 1 further consisting of working the sheets atroom temperature to a desired thickness less than said secondpredetermined thickness.

7. The method of making magnetically isotropic sheets or tapes from amelt containing from 5 to 16 percent aluminum and 95 to 84 percent iron,which consists of casting the melt into relatively thick sheets, hotworking the sheets to a first predetermined thickness, cold working thesheets to a second predetermined thickness less than said firstpredetermined thickness, and thereafter reducing the sheets to a desiredthickness by subjecting the sheets to alternate operations of working atroom temperature and heating to a temperature within the orderdisordertemperature range.

8. A method for making ductile magnetically isotropic sheets whichcomprises producing a melt consisting of 5 to 16 percent aluminum andthe remainder iron, forming the melt into slabs, hot working the slabsabove the recrystallization temperature to a first predeterminedthickness, and working the slabs below the recrystallization temperatureWithin the order-disorder temperature range to any desired thickness.

9. The method of claim 8 wherein the working below the recrystallizationtemperature is performed at an average temperature of 575 C.

10. The method of claim 9 wherein the hot working is performed at anaverage temperature of approximate ly 1000 C.

11. The method of claim 8 wherein the working be low therecrystallization temperature comprises the steps of rolling the sheetsin the order-disorder temperature range to a second predeterminedthickness, maintaining the sheets of said predetermined thickness at atemperature within the order-disorder temperature range for a shortperiod of time, and thereafter reducing the sheets to the desiredthickness.

12. The method of claim 11 wherein the reducing to the desired thicknessis performed by rolling the sheets at room temperature.

13. The method of claim 12 wherein the rolling in the order-disordertemperature range is performed at an average temperature of 575 C. andwherein the temperature at which the sheets are maintained for a shortperiod of time is approximately 575 C.

14. The method for making ductile tape-like magnetic strips whichcomprises producing a melt consisting of essentially 5 to 16 percentaluminum and the remainder iron, forming the melt into slabs, hotworking the slabs above the recrystallizaiton temperature to sheets of afirst predetermined thickness, rolling the sheets in the orderdisordertemperature range to a second predetermined thickness, maintaining thesheets of said second prede termined thickness at a temperature withinthe order-dis order temperature range for a short period of time,cutting the sheets into strips of preselected Widths, and alternatelyrolling and trimming the strips at room temperature to desired widthsand thicknesses.

15. The method of making ductile magnetic sheets and tape-like magneticstrips which comprises producing a melt consisting of essentially 5 to16 percent aluminum and the remainder iron, forming the melt into slabs,hot working the slabs above the recrystallization temperature to sheetsof a first predetermined thickness, rolling the sheets in theorder-disorder temperature range to a second predetermined thicknessless than said first predetermined thickness, maintaining the rolledsheets at a temperature Within the order-disorder temperature range fora short period of time, reducing the sheets to a third predeterminedthickness by subjecting the sheets to alternate operations of rolling atroom temperature and heating to a temperature within the order-disordertemperature range, and thereafter working the sheets at room temperatureto a desired thickness.

References Cited in the file of this patent UNITED STATES PATENTS2,300,336 Bozorth et al Oct. 27, 1942

7.THE METHOD OF MAKIANG MAGNETICALLY ISOTROPIC SHEETS OR TAPES FROM AMELT CONTAINING FROM 5 TO 16 PERCENT ALUMINUM AND 95 TO 84 PERCENT IRON,WHICH CONSISTS OF CASTING THE MELT INTO RELATIVELY THICH SHEETS, HOTWORKING THE SHEETS TO A FIRST PREDETERMINED THICKNESS, COLD WORKINGSHEETS TO A SECOND PREDETERMINED THICKNESS LESS THAN SAID FIRSTPREDETERMINED THICKNESSS, AND THEREAFTER REDUCING THE SHEETS TO ADESIRED THICKNESS BY SUBJECTING THE SHEETS TO ALTERNATE OPERATIONS OFWORKING AT ROOM TEMPERATURE AND HEATING TO A TEMPERATURE WITHIN THEORDERDISORDER TEMPERATURE RANGE.